Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 74.5	$ 77.5	$ 200.3	$ 249.7
Segment Profit	2.5	1.7	13.4	15.3
Capital Expenditures	28.3	25.3	39.6	38.4
Segment Assets	1,051.7	993.9	1,051.7	993.9	$ 1,038.8
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	28.9	27.6	102.0	127.9
Segment Profit		(0.7)	9.4	10.9
Capital Expenditures	18.8	15.6	21.9	22.8
Segment Assets	593.8	573.8	593.8	573.8	590.9
Electricity
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	44.1	48.4	95.2	118.7
Segment Profit	2.1	1.9	3.4	3.7
Capital Expenditures	7.3	7.3	13.0	11.7
Segment Assets	419.4	406.5	419.4	406.5	415.1
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit	0.1	0.2		0.1
Capital Expenditures	2.2	2.3	4.7	3.8
Segment Assets	31.5	7.6	31.5	7.6	26.3
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	1.5	1.5	3.1	3.1
Segment Profit	0.3	0.3	0.6	0.6
Capital Expenditures		0.1		0.1
Segment Assets	$ 7.0	$ 6.0	$ 7.0	$ 6.0	$ 6.5

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(12))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Investing Activities
-URI http://asc.fasb.org/extlink&oid=6516133

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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